CVE-2026-86205: h3 before 2.0.1-rc.18 Open Redirect via redirectBack()

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-06 · updated 2026-09-08
Weakness: CWE-601

Description

h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.

Affected products

h3js h3: from 2.0.1-rc.17 before 2.0.1-rc.18 — fixed in 2.0.1-rc.18

How to fix

Upgrade to a fixed version: h3js h3 2.0.1-rc.18. Check the vendor advisory for mitigations.

References

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